In the application of solid particulate material, such as powdered paints in industrial finishing applications, the powder material is commonly conveyed to a spray gun by pressure and then dispensed from a spray nozzle attached to the gun as a powder entrained, air stream towards an object or target substrate to be coated or painted. The particles of powder being dispensed from the gun are imparted with an electrical charge to electrostatically attract them towards the substrate held at electrical ground potential. After coating, the target substrate is usually conveyed into an oven for heating and melting the powder coating material on the substrate.
A powder spray gun is generally constructed from a barrel, formed with a powder flow passage, having a spray nozzle mounted at the forward end thereof. The spray nozzle can be formed with a generally circular-shaped discharge opening, as disclosed in U.S. Pat. Nos. 3,659,151, 4,380,320, 4,811,898 and 5,056,720, through which powder coating particles are emitted to form a generally conical shaped spray pattern upon the substance to be coated. Alternatively, the spray nozzle can be formed with a generally elongated, rectangular-shaped slot, of the type disclosed in U.S. Pat. Nos. 3,659,787, 4,638,951 and 4,830,279 as well as European Patent Publication No. 0237207, through which the powder coating particles are discharged to form a so-called "flat" spray pattern which diverges outwardly in a generally triangular shape. Another spray nozzle could be formed with cross cut slots disposed at ninety degrees to each other, as illustrated in FIGS. 3-5, herein.
In order to maximize coverage of a target substrate with coating particles, a strong electrostatic field can be created between an electrode and the target substrate so the coating particles are adequately charged and then strongly attracted to the target substrate. Typically, an electrode is mounted at the forward end of the spray gun, in the vicinity of the discharge opening of the spray nozzle, to impart the electrostatic charge to the powder coating particles emitted from the spray nozzle.
While the prior art nozzle designs have been generally adequate for spraying substrates or products with primarily flat, planar surfaces, they have been less effective for spraying an even coating of powder on substrates, such as the product 8 illustrated in FIG. 1 having a recess or corner configuration like triangularly shaped groove 10 with side walls 12 and 14 which converge and intersect at an innermost edge 16. Moreover, prior art nozzles have been generally ineffective for spraying both a flat surface as well as a recess or corner configuration with an even coating of powder. This deficiency is particularly accentuated in a production line where an operator needs the flexibility of spraying the moving surfaces of advancing substrates have various configurations without the need of changing spray guns or nozzles.
When spraying with nozzles described in certain patents listed before, the problem of forming an uneven coating problem is sometimes caused by the Faraday cage effect. This is an electrostatic phenomena where charged coating particles looking for ground, i.e., the object being sprayed, are more attracted to and therefore stick to the outer sections 18 and 20 of side walls 12 and 14, as illustrated in FIG. 2, respectively, prior to coating the innermost section 22 adjacent the innermost edge 16. The result is an uneven coating with the innermost section 22 having a lighter coating of powder than the more outwardly positioned sections 18,20 of the sidewalls.
To overcome this deficiency, various types of prior art nozzles have been used to spray a recess in a target substrate. For example, a small, tubular shaped spray pattern formed with a pattern adjustment sleeve incorporated in the spray gun described in the previously listed U.S. Pat. No. 4,811,898 patent, can be moved forward relative to a small deflector to turn the flow in a more forward direction. This setup narrows the width of the spray pattern of coating particles applied to the object being coated. While the coating material can then penetrate deep enough into the recess, the spray pattern has a doughnut shape with the powder coating in the center being lighter than the outer part of the pattern. Therefore, the powder still has a tendency to stick to the sidewalls without penetrating enough to evenly cover the sidewalls of the innermost section of the recess.
Alternatively, the deflector of the gun described in the U.S. Pat. No. 4,811,898 patent can be positioned so that a "pin point" spray pattern reaches the innermost portion of the recess. However, with this nozzle configuration, the powder does not adequately coat the outer sections of the recess sidewalls. Since the parts or substrates being sprayed are typically moving down a paint line at a relatively rapid rate of about ten to thirty feet a minute, the operator does not have adequate time to properly spray both the innermost and outer sections of the sidewalls forming the recess.
Spray nozzles with ninety degree, cross cut slots overcome this problem and the powder can evenly coat a small section of the sidewalls in the recess, as illustrated in FIG. 2A. However, the powder still does not adequately coat the sidewalls of a deep recess. Further, this type of nozzle is not suitable for applying a large, even spray pattern on the adjacent flat surfaces of the substrate.
With the flat spray pattern produced by nozzles described in certain other patents listed before, the coating powder is pressurized and emitted from the nozzle at a high velocity so that the discharged particles cover a wide spray pattern. While effective for coating a flat surface, the high pressure causes the powder to rebound out of a recess. Under these circumstances, the flow direction of the spray pattern is not adequately controlled and the resulting spray coating is uneven within the recess. To compensate for the uneven coating, excess powder can be sprayed to cover the sides of the recess. However, this is detrimental because of the increase in powder usage and the resulting added manufacturing costs. Another problem relating to increasing the pressure and/or velocity of the powder emitted from the nozzle is an increase in the wear on the spray gun parts and the additional expense in replacement parts and operator time to maintain the spraying equipment.
Another problem caused by increasing the velocity of charged powder flow is the shortened time in which the powder resides in the strongest part of the electrostatic field which is nearest to the electrode which results in a decrease in electrostatic charge applied to the particles. The result is that the powder penetrates deeper into the recess without adequately sticking to the side walls.